1. Field of the Invention
This invention relates to a leakage test method and apparatus for determining, using air or other gas, whether there is any flaw in a hollow object such as a
2. Prior Art
Conventional air leakage test methods are generally classified into the simplified type and the pressure differential-detecting type.
A leakage test method of the simplified type as shown in FIG. 1 of Japanese Laid-Open (Kokai) Patent Application No. 49774/76 is performed using a leakage test apparatus including an air passage having a pressure input terminal and a pressure output terminal, the pressure input terminal being connected to a pressure source. A regulator, a three-port exhaust valve, and an on-off valve are provided on the air passage in this order from the upstream side to downstream side of the air passage, and a pressure sensor is connected to the air passage at a position downstream of the on-off valve.
In the above leakage test method of the simplified type, the on-off valve is set in an open condition, and the exhaust valve is set in such a manner that the pressure output terminal is disconnected from the regulator and communicates with the ambient atmosphere. In this condition, a hollow object to be tested (hereinafter often referred to as "object") is connected to the pressure output terminal. Then, the exhaust valve is switched to connect the object to the regulator. As a result, the pressure of the pressure source is reduced by the regulator, and this reduced pressure is applied to the object as a test pressure. Then, the on-off valve is closed to disconnect the object from the regulator, and in this condition, the pressure sensor determines whether any leakage develops in the object, that is, whether the pressure within the object is lower than the test pressure.
In the above leakage test method of the simplified type, when the test pressure is introduced into the object, the temperature of the air in the object becomes higher than the ambient temperature because of the compression of the air. Therefore, when the object is disconnected from the pressure source immediately after the test pressure is introduced into the object, the temperature within the object drops because of heat radiation from the object. As a result, even when there is no air leakage in the object, the air pressure within the object gradually drops. This pressure drop stops when the temperature of the air within the object becomes equal to the ambient temperature.
In the above method, the leakage detection can not be carried out a short period of time after the test pressure is introduced into the object. The reason is that the air pressure drop within the object due to the above-mentioned heat radiation can not clearly be distinguished from the air pressure drop due to an air leakage developing in the object. Thus, it is difficult to clearly judge whether there is any leakage in the object.
Therefore, practically, the communication of the object with the regulator is maintained until the temperature within the object becomes equal to the ambient temperature, thereby maintaining the pressure within the object at the test pressure. Then, the on-off valve is closed, and the leakage detection is carried out. With this method, however, since the leakage detection must wait until the temperature within the object becomes equal to the ambient temperature, the time required for one cycle of the leakage test is long, and therefore this leakage test can not be carried out efficiently.
In an apparatus for performing an air leakage test method of the pressure differential-detecting type, as disclosed in Japanese Laid-Open Utility Model Application Nos. 66486/74 and 99686/79, an air passage has two branch passages at the downstream side, and the distal ends of the two branch passages serve as pressure output terminals, respectively. A regulator and an exhaust valve are provided on the air passage in this order from the upstream side, and on-off valves are provided on the two branch passages, respectively. Each of two pressure inlet ports of a pressure sensor is connected to a respective one of the two branch passages at a position downstream of the on-off valve. The exhaust valves and the on-off valves perform the same functions as those described above for the leakage test method of the simplified type. An object to be test and a reference container confirmed to be free from leakage are connected to the two pressure output terminals, respectively. A test pressure is applied to both the object and the reference container. The pressure sensor detects a pressure differential between the pressure within the object and the pressure within the reference container. More specifically, when the object is not subjected to pressure leakage, the pressures inputted respectively into the pressure inlet ports of the pressure sensor are equal, and therefore an output voltage of the pressure sensor is substantially zero. When there is a pressure leakage in the object, the pressure within the object is lower than the pressure within the reference container, and therefore the pressure sensor feeds an output voltage representative of such a pressure difference.
With the above air leakage test method of the pressure differential-detecting type, the leakage can be detected with a high sensitivity. In addition, when the object and the reference container have the same volume and the same shape, the airs within the object and the reference containers are subjected to the same pressure drop due to a heat radiation. Therefore, it is not necessary for the leakage detection to wait until the temperature of the air within the object and the reference container becomes equal to the ambient temperature. Actually, however, in many cases, the reference container is different in volume and shape from the object. In such a case, the reference container is different in heat radiation from the object, and hence different in pressure drop due to such heat radiation. Therefore, when the object and the reference container in their sealed condition radiate heat, there occurs a difference in pressure between the two. Therefore, the leakage detection must wait until the temperature of the air within each of the object and the reference container becomes equal to the ambient temperature. Thus, with this conventional method, the leakage test can not be carried out in an efficient manner.
The prior art related to the present invention is disclosed in Japanese Laid-Open Patent Application Nos. 60586/74, 53095/74 and 48385/76. Particularly, Japanese Laid-Open Patent Application No. 60978/74 and FIG. 2 of Japanese Laid-Open Patent Application No. 49774/76 disclose the technique of stabilizing the pressure, but a pressure relieving means is not disclosed.